Check whether a destination is available before dialling

Opens a `text/event-stream` (SSE) connection that pushes call lifecycle transitions (ringing, answered, completed, failed, …) as they happen. Intended for the in-app dialer's live call view. Past lifecycle events are also embedded inline on the Call resource (`Call.events`) — this endpoint is only for the live tail; there is no separate read endpoint for historical events. For a call handled by an AI agent the same stream also carries the live transcript, as two additional event kinds: - `transcript.turn` — a complete, flushed turn (`{kind, call_id, seq, role, text, final, occurred_at}`). The durable copy of the same turns is on `GET /v1/calls/{id}?transcript=1`. - `transcript.delta` — an incremental chunk of the turn being spoken right now (`delta` instead of `text`). Stream-only: partials are never persisted. The stream opens with a `: ready` SSE comment the moment it attaches — useful as a connection check, and ignorable by any conforming SSE client. A subscriber that joins mid-call is replayed the last 20 transcript events so it does not sit silent until the next turn. Neither transcript kind is available as a workspace webhook — they are per-call and far too chatty for HTTP fan-out. Read them here, with an API key (`calls.read`) like any other call read. Note that a browser `EventSource` cannot send an `Authorization` header, so an API-key subscriber must be server-side (curl, `fetch` with a reader, or any SSE client that sets headers). The connection stays open until the call reaches a terminal state or the client disconnects. The two id forms behave slightly differently by design. A session id (`HD_…`) attaches immediately — the browser dialer knows it before the call row exists, and subscribing early is the point, so no existence check is made; the channel is always scoped to the calling workspace, so an unknown session id simply yields a stream that never emits. A call UUID is looked up, and 404s when this workspace owns no such call. A call whose PBX session has not landed yet holds the request open for a few seconds rather than failing, since origination returns the UUID before the box calls back.